The present invention relates to an improved process for producing oxygen in a two bed vacuum swing adsorption (VSA) gas separation process.
Cyclic adsorption processes are frequently used to separate the components of a gas mixture. Typically, cyclic adsorption processes are conducted in one or more adsorbent vessels that are packed with a particulate adsorbent material which adsorbs at least one gaseous component of the gas mixture more strongly than it adsorbs at least one other component of the mixture. The adsorption process comprises repeatedly performing a series of steps, the specific steps of the sequence depending upon the particular cyclic adsorption process being carried out.
In any cyclic adsorption process, the adsorbent bed has a finite capacity to adsorb a given gaseous component and, therefore, the adsorbent requires periodic regeneration to restore its adsorption capacity. The procedure followed for regenerating the adsorbent varies according to the process. In VSA processes, the adsorbent is at least partially regenerated by creating vacuum in the adsorption vessel, thereby causing adsorbed component to be desorbed from the adsorbent, whereas in PSA processes, the adsorbent is regenerated at atmospheric pressure. In both VSA and PSA processes, the adsorption step is carried out at a pressure higher than the desorption or regeneration pressure.
A typical VSA process generally comprises of a series of four basic steps that includes (i) pressurization of the bed to the required pressure, (ii) production of the product gas at required purity, (iii) evacuation of the bed to a certain minimum pressure, and (iv) purging the bed with product gas under vacuum conditions. In addition a pressure equalization or bed balance step may also be present. This step basically minimizes vent losses and helps in improving process efficiency. The PSA process is similar but differs in that the bed is depressurized to atmospheric pressure and then purged with product gas at atmospheric pressure.
As mentioned above, the regeneration process includes a purge step during which a gas stream that is depleted in the component to be desorbed is passed countercurrently through the bed of adsorbent, thereby reducing the partial pressure of adsorbed component in the adsorption vessel which causes additional adsorbed component to be desorbed from the adsorbent. The nonadsorbed gas product may be used to purge the adsorbent beds since this gas is usually quite depleted in the adsorbed component of the feed gas mixture. It often requires a considerable quantity of purge gas to adequately regenerate the adsorbent. For example, it is not unusual to use half of the nonadsorbed product gas produced during the previous production step to restore the adsorbent to the desired extent. The purge gas requirement in both VSA and PSA processes are optimization parameters and depend on the specific design of the plant and within the purview of one having ordinary skill in the art of gas separation.
Many process improvements have been made to this simple cycle design in order to reduce power consumption, improve product recovery and purity, and increase product flow rate. These have included multi-bed processes, single-column rapid pressure swing adsorption and, more recently, piston-driven rapid pressure swing adsorption and radial flow rapid pressure swing adsorption. The trend toward shorter cycle times is driven by the desire to design more compact processes with lower capital costs and lower power requirements. The objective has been to develop an adsorbent configuration that demonstrates an ability to produce the required purity of oxygen, with minimum power consumption and lower capital costs.
The present invention provides for a process for separating oxygen from nitrogen utilizing a vacuum swing adsorption process. The process utilizes two beds, an air blower, a vacuum pump and a product reservoir. The process employs ambient air to pressurize a bed while the bed pressure is lower than atmospheric pressure. The process depressurizes a bed to atmosphere while the bed pressure is higher than ambient pressure. The process also employs a two step bed purge using the product gas. The process maximizes the air blower and the vacuum pump utilization to achieve higher productivity and lower power consumption.
In a first embodiment of the invention, the two bed cycle is a process for selectively separating oxygen from a gas stream in two adsorbent beds containing an adsorbent selective for nitrogen and a product reservoir. The process comprises the following five steps:
(a) An air stream is added to the first bed with an air blower and/or ambient air. The first bed being at a lower pressure from the previous cycle also receives oxygen product gas from the second bed. The second bed is at a higher pressure from the same previous cycle and is regenerated by removing waste gas through a vacuum pump.
(b) The air stream is added to the first bed with the air blower while the waste stream is removed from the second bed with the vacuum pump.
(c) The oxygen product is withdrawn from the first bed while air is still being added to it with the air blower. The waste stream is still being removed from the second bed with the vacuum pump.
(d) A portion of the oxygen product withdrawn from the first bed is diverted from the product reservoir to the second bed while the air blower is adding air to the first bed and the waste stream is being removed from the second bed with the vacuum pump.
(e) The oxygen product from the first bed is fully diverted from the product reservoir to the second bed. The first bed is also being depressurized to atmosphere while the air blower discharge pressure is ramped down. The waste stream is removed from the second bed via the vacuum pump.
(f) Repeat steps (a) through (e) by reversing the operating modes of the first bed and second bed so that the second bed will be the production bed and the first bed the regeneration bed.
The present inventors anticipate that this inventive process will not only be useful in vacuum swing adsorption (VSA) processes but also pressure swing adsorption (PSA) processes as well.
The nitrogen adsorbent material is any adsorbent that is capable of preferentially adsorbing nitrogen over oxygen. Examples include zeolite, such as zeolite X, molecular adsorption sieves. Preferably the zeolite X sieve is a lithium ion-exchanged type X zeolite, having a silicon to aluminum atomic ratio in the zeolite lattice of between 0.9 and 1.1. Preferably, this range is from 1.0 to 1.1 with a ratio of silicon to aluminum less than 1.08 most preferred for the type X zeolite. Of the available exchangeable cation sites on the type X zeolite, preferably at least 50% are occupied by ions from Groups 1A, 1B, 2A, 2B or mixtures of these. Of these groups, sodium, lithium, potassium, silver(I), calcium, strontium and mixtures of these are the most preferred cations. The type X zeolite may also be comprised of lithium or lithium and bivalent cation or lithium and a trivalent cation. Preferably about 50% to about 95% of the available exchangeable sites are occupied by lithium ions and about 50 to 5% are occupied by (a) divalent cations selected from the group consisting of calcium ions, strontium ions, magnesium ions, barium ions, zinc ions, copper (II) ions and mixtures of these, (b) trivalent ions selected from the group consisting of aluminum, scandium, gallium, iron (II), chromium (III), indium, yttrium, single lanthanides, mixtures of two or more lanthanides and mixtures of these or (c) combinations of (a) and (b).
The air stream that is treated may contain oxygen and nitrogen in any proportion but preferably is about 21% oxygen and 79% nitrogen.
Typical operating conditions for this process are maximum feed pressures greater than 1013 millibar and minimum vacuum pressures of up to 260 millibar or higher. Partial cycle times of between of about 10 to 60 seconds are typically employed in a VSA cycle. Typical operating conditions for PSA are feed pressure up to 3 bara with a cycle time of between about 5 to 60 seconds.